1. Field of the Invention
The present invention relates to an optical guide and an image forming apparatus which uses the optical guide in an eraser or an auxiliary transfer device.
2. Description of the Related Art
FIG. 1 schematically shows an example of a related image forming apparatus. The conventional image forming apparatus includes a charger including a charging roller 1, a laser scanning unit (LSU) 20, and a photoreceptor drum 3 as a photosensitive medium. The conventional apparatus further includes a developing unit including a developing roller 5, a transfer unit including a transfer roller 9, and an auxiliary transfer device 10 which increases the surface potential of the photoreceptor drum 3 by radiating light on the surface of the photoreceptor drum 3 so that a transfer efficiency of a toner image is improved. Also, the image forming apparatus further includes an eraser 2 which initializes the surface potential of the photoreceptor drum 3. Here, the auxiliary transfer device 10 may include a transfer belt instead of the transfer roller 9.
In the image forming apparatus having the above structure, the toner image is formed on the surface of the photoreceptor drum 3 and transferred onto a paper 8 as described below.
The surface of the photoreceptor drum 3 is charged by the charging roller 1 to a predetermined voltage and has a predetermined negative surface potential. Thus, when a laser beam is irradiated by the laser scanning unit (LSU) on a portion where the toner image on the surface of the photoreceptor drum 3 is to be formed, the surface potential of the photoreceptor drum 3 in the portion on which the laser beam is irradiated varies, and thus, an electrostatic latent image is formed. When the electrostatic latent image is developed by the developing roller 5, toner is attached to the electrostatic latent image, and thus, the toner image is formed. The toner image formed on the surface of the photoreceptor drum 3 is transferred by the transfer roller 9 to the paper 8.
In the image forming apparatus, for example, a DC voltage of about −1400V is applied to the charging roller 1, and a DC voltage of about −200V is applied to the photoreceptor drum 3 such that the surface of the photoreceptor drum 3 is charged and the surface potential of the photoreceptor drum 3 is set to about −800V. In addition, an exposure potential by the LSU 20 (i.e., the surface potential of the photoreceptor drum 3 in the portion on which the laser beam is irradiated by the LSU, which corresponds to the potential of the electrostatic latent image) is set to −50V, and the surface of the developing roller 5 is charged to about −300V. Thus, toner from the developing roller 5 is attached to the electrostatic latent image having the exposure potential. In addition, the transfer roller 9 is charged to +1200V such that the toner image on the surface of the photoreceptor drum 3 is transferred to the paper 8.
The auxiliary transfer device 10, which is a so-called pre-transfer lamp (PTL), improves an efficiency with which the toner is transferred from the surface of the photoreceptor drum 3 on the paper 8 and is installed between the developing roller 5 and the transfer roller 9. The auxiliary transfer device 10 is turned on at a specific time and light is irradiated on the surface of the photoreceptor drum 3 so that the charging potential of the surface of the photoreceptor drum 3 is reduced by half. For example, the charging potential is reduced to about −400V, such that a difference between the exposure potential by the laser beam irradiated by the LSU 20 and the surface potential of the surface of the photoreceptor drum 3 around the exposure potential is decreased. As such, the toner attached to a position corresponding to the exposure potential of the surface of the photoreceptor drum 3 is easily transferred. Thus, the toner on the surface of the photoreceptor drum 3 can be easily transferred even at a low transfer voltage. In this case, since the toner is attached to the position of the exposure potential, light emitted from the auxiliary transfer device 10 is not irradiated on the position of the exposure potential, and thus, the exposure potential is not changed.
The auxiliary transfer device 10 is provided to radiate light having a wavelength analogous to that of the laser beam emitted from the LSU 20, for example, having a wavelength of about 600-800 nm.
FIG. 2 schematically shows a conventional auxiliary transfer device for an image forming apparatus. Referring to FIG. 2, the auxiliary transfer device for a conventional image forming apparatus includes a light emitting diode (LED) array 11 and a lens 15 which causes light output from the LED array 11 to condense on a photoreceptor drum 3.
The LED array 11 includes twelve to eighteen LEDs that are arranged in a row in a lengthwise direction of the photoreceptor drum 3 to uniformly irradiate light on an entire surface of the photoreceptor drum 3 in the lengthwise direction, i.e., a main scanning direction of the photoreceptor drum 3.
However, round-type LEDs of the same specification might differ in brightness by as much as 30%. Thus, when the LED array 11 in which the twelve to eighteen LEDs are arranged in a row is used as described above, unless LEDs outputting the same amount of light are sorted and applied, a difference in the amount of output light for the LEDs is large, and thus, a print image concentration becomes nonuniform.
That is, transfer failure occurs in a portion on which light is irradiated from an LED having a very small amount of output light, and a concentration difference of block color occurs in a fine image in a portion on which light is irradiated from an LED having a very large amount of output light, and thus, printing quality is lowered. The concentration difference of block color occurs when the amount of irradiated light is strong, since a difference between the exposure potential and the charging potential is decreased so that the toners are not concentrated on the position of the exposure potential and are scattered around printed dots. As a result, the dots appear too large and thus are darker in a gray image.